1. Any large division of a building lying across its main axis at 90°. In an Early Christian basilica it was the large and high structure to the liturgical west of the apse, on occasion so high that the nave and aisles stopped against its wall, as in the C4 Constantinian basilica of San Pietro, Rome.
2. In a cruciform church the transept is often of the same section as the nave, and may have no aisles, or one, or two (called cross-aisles): eastern transept-aisles were usually subdivided into chapels. At the position where the transepts branched on either side of the crossing, often marked by a crossing-tower (e.g. Lincoln Cathedral), flèche (e.g. many French cathedrals), or lantern (e.g. Ely Cathedral), the choir continued eastwards, often divided immediately to the east of the crossing by a pulpitum or choir-screen. Larger medieval cathedrals (e.g. Lincoln) sometimes had secondary transepts at the west end of the nave (really a form of narthex), and to the east of the crossing, on either side of the sanctuary and choir-aisles: in both cases they would have had eastern chapels.
Jane Turner (1996)
tran·sept / ˈtranˌsept/ • n. (in a cross-shaped church) either of the two parts forming the arms of the cross shape, projecting at right angles from the nave: the north transept.DERIVATIVES: tran·sep·tal / tranˈseptl/ adj.